The present invention relates to a light source device that can be applied to a backlighting arrangement of a liquid-crystal display or the like and further relates to a light regulator or light regulator member (hereinafter referred to simply as "light regulator") for use with the light source device. Surface light source devices for liquid-crystal display backlighting applications include a type in which the primary light source is provided at the rear surface of a plate-shaped optical member having a surface from which the illuminating light is emitted, and a type in which the primary light source is provided at a side surface of the plate-shaped optical member. The latter type is termed a side-light type surface light source device. In both types of surface light source device, a light regulator in the form of a prism sheet is disposed usually on the light emission surface of the plate-shaped optical member to correct the directivity of the illuminating light.
Side-light type surface light source devices include a plate-shaped optical member that functions as a light guide plate, and a rod-shaped primary light source arranged at the side of the plate-shaped optical member. Light from the primary light source is deflected by the light guide plate and emitted from the major surface of the light guide plate to be used for backlighting of, for example, a liquid-crystal panel. Since the primary light source is arranged at the side of the light guide plate, side-light type surface light source devices generally have the advantage of being very thin.
Types of light guide plates employed in side-light surface light source devices include those that are substantially uniform in thickness, and those having a tendency to decrease in thickness going away from the primary light source. The latter type generally can emit light more efficiently than the former type.
FIG. 9 is a disassembled perspective view of a conventional side-light surface light source device of the latter type. With reference to FIG. 9, the side-light surface light source device 1 includes a light guide plate 2, a primary light source 3 disposed alongside the light guide plate 2, a reflection sheet 4, and a light regulator in the form of a prism sheet 5. The reflection sheet 4, light guide plate (plate-shaped optical member) 2 and prism sheet 5 are stacked to form a laminated arrangement. The light guide plate 2 is, for example, a light scattering guide plate made of light scattering and guiding material. The primary light source 3 has a cold-cathode tube or fluorescent lamp 6, and a regular reflector 7 around part of the circumference of the lamp 6. Scattered light from the reflector 7 impinges on edge T of the light guide plate 2.
The reflection sheet 4 is a sheet-like regular reflection member of metal foil or the like, or a sheet-like irregular reflection member of white PET film or the like. Light that leaks from the light guide plate 2 is reflected back to the light guide plate 2 by the reflection sheet 4. The light guide plate 2, which has a wedge-shaped cross-section, has an internal scattering power. The light guide plate 2 is comprised of matrix of, for example, polymethyl methacrylate (PMMA) which contains a uniform distribution of light-permeable particles having a different refractive index from that of the matrix material.
With reference to FIG. 10, which is a sectional view along line A--A of FIG. 9, the edge of the light guide plate 2 near the primary light source 3 provides the light incidence surface T. Light L from the primary light source 3 enters the light guide plate 2, where it is scattered by the particles and reflected by the reflection sheet 4. When the reflection sheet 4 is an irregular reflector, the light is subjected to irregular reflection. The light L is propagated in a series of repeated reflections repeatedly between the (sloping) surface near the reflection sheet 4 and the surface near the prism sheet 5 (hereinafter referred to as "exit surface"). In this specification, the surface near the reflection sheet 4 will be referred to as the "sloping surface," and the surface near the prism sheet 5 will be referred to as the "exit surface."
In the course of this propagation, each time the light L is reflected by the sloping surface, the angle of incidence of the light relative to the exit surface decreases. Also, each time the light impinges on the exit surface, the component that forms an angle of incidence relative to the exit surface equal to or below the critical angle is emitted from the exit surface. Light L that exits from the exit surface is light that has been scattered by the particles in the light guide plate 2 and irregularly reflected by the reflection sheet 4. However, as is well known, the principal direction of the emitted light L is inclined toward the end of the wedge. Thus, the light L is not emitted in random directions, but with quite some directivity. This characteristic of the light guide plate 2 and side-light surface light source device 1 will be referred to as the "directional emission characteristics." This directivity is in a plane perpendicular to the lamp 6.
The prism sheet 5 is formed of a light-permeable sheet of polycarbonate or the like, with prisms on both surfaces. In addition to correcting the above directivity, the prism sheet 5 can also serve to correct the directional propagation characteristics of the light in a longitudinal plane of the lamp 6, as required. The prism surfaces are each provided with an array of projections each comprised of a pair of inclined faces to form a triangular cross-section, with the projections on one surface being arrayed perpendicularly to the projections on the other surface. For example, the projections on the inner prism surface are aligned substantially parallel to the light incidence surface T of the light guide plate 2, as shown in the enlarged inset indicated by arrow B, while the projections on the outer prism surface run perpendicular to the light incidence surface T, as shown in the enlarged inset indicated by arrow C. In the description given herein, the orientation parallel to the surface T is referred to as the X-direction and that perpendicular to the surface T is referred to as Y-direction.
With respect to Y-direction, the projections on the inner side of the prism sheet 5 correct the principal direction of the emitted light L to the front, while with respect to the X-direction, the projections on the outer side reduce light divergence. If required, the angle of the sloping surface of the prism sheet can be adjusted to provide the side-light surface light source device 1 with a desired directivity. It should be noted that light components impinging on the prism sheet 5 at or above the critical angle are reflected back into the light guide plate 2 to be reutilized in the generation of illuminating light. A single-sided prism sheet may be used. In general, a side-light surface light source device 1 that uses a wedge-shaped light guide plate and prism sheet as above can be expected to output light to the front more efficiently than a side-light surface light source device having a light guide plate of substantially uniform thickness.
Instead of the light-permeable particles described above, there is known the use of a semi-transparent light scattering guide plate comprised of transparent resin in which particles of silica or the like are distributed. It should also be noted that a light guide plate with directional emission characteristics may be used without employing a scattering guide plate. For example, a light guide plate formed of a transparent member with a matted exit surface and/or sloping surface has directional emission characteristics. Similarly, a light guide plate having directional emission characteristics can be obtained by using a transparent light guide plate with a micro-lens array, scattering layer or the like on the exit surface and/or sloping surface. While there will be some difference, employment of a prism sheet to correct the directivity, as in the case of the side-light surface light source device described with reference to FIG. 9, will provide an efficient output of light to the front, even with such a guide plate.
When a side-light surface light source device that has a flat-plate-shaped light guide plate, matting, a micro-lens array or a light scattering layer or the like is provided on the exit surface and/or the other surface. The flat-plate-shaped light guide plate replaces the light guide plate 2 shown in FIG. 9. In such a case, the prism sheet can be used to correct the directivity of the emitted light through selective transmission of the illuminating light emitted as scattered light from the exit surface of the guide, so that the emitted light is concentrated to the front.
Various directivity characteristics will be desired with the above-described various types of side-light surface light source devices, depending on the devices to be applied to. As such, it is necessary to adjust the correctional function of the prism sheet in accordance with the directivity required. This has to be done by adjusting the design values of the angles of the prism sloping and/or the refractive index of the material. However, such methods are subject to limits in adjusting directivity. Moreover, there is little difference between design changes involving only one prism surface and design changes that involve the whole prism sheet. Even if, for example, it is only necessary to alter the directivity adjustment capability in the X-direction, a new mold still has to be made up to form the whole prism sheet. In addition, time is required to check the characteristics of the newly-fabricated prism sheet.
Viewing from another standpoint, with this type of prism sheet, the ends of the projections on the exit surface tend to be deformed, reducing the inclined-face proportion, as shown in FIG. 11. This can degrade the directivity and give rise to adhesion with the guide plate. This deformation can be prevented by using a transparent resin having a high thermosoftening point. However, the refractive index of such resin will limit the characteristics of the prism sheet and produce a further decrease in the range of adjustability.
The use of a lamination of single-sided prism sheets instead of a double-sided prism sheet can be expected to resolve these drawbacks. A lamination of single-sided sheets also enables directivity correction in X- the and Y-directions to be adjusted independently, by changing the combination of sheets employed. The result is that illuminating light of a desired directivity can be more easily obtained than when a double-sided prism sheet arrangement is employed. Moreover, the range of adjustability can be expanded by combining single-sided prism sheets having different refractive indexes. Also, degradation in directivity caused by deformation of the projections can be prevented by forming one of the single-sided prism sheets of transparent resin having a high thermosoftening point.
A problem that does arise when a lamination of single-sided prism sheets is employed is that minute spaces between the laminations can produce Newton's rings, degrading the quality of the emitted light. Also, long-term use and other factors can cause partial adhesion of single-sided prism sheets to the exit surface, giving rise to unnatural patterns or fringes. One way of resolving such problems is to optically bond the single-sided prism sheets. However, when a sheet such as a single-sided prism sheet that is formed as a repeated configuration in one direction and supported by bonding exhibits a thermal contraction in the direction of the projections that is different from the thermal contraction perpendicular thereto. Therefore, a heat cycle can produce cloudiness of bonded surfaces, reducing the quality of the emitted light.